High thermoelectric performance of superionic argyrodite compound Ag₈SnSe₆

Abstract

A good thermoelectric material usually has a high power factor and low thermal conductivity for high figure of merit (ZT), and is also environmentally friendly and economical. Superionic compounds are heavily studied because of their ultra-low thermal conductivity even though the thermal stability remains an issue. In this work, we report a superionic argyrodite compound Ag₈SnSe₆ as a promising mid-temperature thermoelectric material because of its high ZT and good thermal stability up to 550 °C. It is revealed that Ag₈SnSe₆ exhibits a decent Seebeck coefficient (n-type) and electrical conductivity. At the same time, its thermal conductivity is lower than the glass limit with the thermal capacity Cᵥ below 3NkB at high temperature, where N is the Avogadro's number and kB the Boltzmann constant. Detailed microstructural and thermodynamic characterization of this compound is performed to understand the electronic and phononic origins of the thermoelectric properties. The highly random ionic occupations in the cubic phase, leading to the molten silver sublattice and phononic mode softening, are responsible for the very low thermal conductivity and ZT ∼ 1.1 at 450 °C.